1. Field of the Invention
The present invention relates to an image projection apparatus such as a projector having a shutter mechanism for hiding a projection image.
2. Description of the Related Art
In many cases, a projector which is used in a meeting or the like repeats displaying (projection) and hiding (non-projection) images in a short time. In the projector, a high-brightness discharge lamp is commonly used as a light source, but the lifetime of the high-brightness discharge lamp gets shorter if turning on and off is repeated in a short time. The high-brightness discharge lamp needs a certain amount of time until its brightness or color is stabilized after the lamp is turned on, and also needs a certain amount of cooling time from the time of turning off the lamp to the time of turning on the lamp again.
Japanese Patent Laid-Open No. 58-21726 discloses a projector which disposes a shutter mechanism between a lamp and a projection lens to open and close it to switch displaying and hiding an image so that displaying and hiding the image can be repeated in a short time without shortening the lifetime of a high-brightness discharge lamp.
The temperatures of an optical element and a member holding it inside the projector increase by receiving light from the lamp while the lamp is lighting. Because of the temperature increase, the optical characteristic, for example refractive index, of the optical element may be changed or the holding member which holds the optical element is thermally expanded and the interval between adjacent optical elements may be changed. The change of the optical characteristic or the interval of the optical elements causes the change of the focus state of the projection image.
Japanese Patent Laid-Open No. 2000-19648 discloses a projector which disposes a temperature sensor inside a chassis and performs a focus correction operation of a projection lens in accordance with the temperature detected by the temperature sensor.
In the projector disclosed in Japanese Patent Laid-Open No. 58-21726, there is a difference of the increase way or the temperature distribution of the optical element or the holding member which holds it between states in which the shutter mechanism (hereinafter, referred to simply as a shutter) is open and closed. This is because the temperature increase does not occur in the optical element and the holding member disposed at a screen side with respect to the shutter, and the temperatures of the optical element and the holding member at a light source side with respect to the shutter additionally increases by the light reflected on the shutter. Therefore, in the projector disclosed in Japanese Patent Laid-Open No. 58-21726, the way of the change of the focus state is different depending upon the open and the close of the shutter. However, Japanese Patent Laid-Open No. 58-21726 does not disclose a corrector that corrects such focus variations.
Japanese Patent Laid-Open No. 2000-19648 does not disclose the relationship between the operation of the shutter and the correction of the focus variation.
There is a projector which performs an AF (autofocus) using reflected light from the projection image, but the focus variation cannot be corrected by using the AF because an image is not projected in a state where the shutter is closed.